[MM-52955] Fix panic for not found posts (#23561)

server/channels/store/sqlstore/reaction_store.go

@@ -41,6 +41,11 @@ func (s *SqlReactionStore) Save(reaction *model.Reaction) (re *model.Reaction, e
 		if err != nil {
 			return nil, errors.Wrap(err, "failed while getting channelId from Posts")
 		}
+
+		if len(channelIds) == 0 {
+			return nil, store.NewErrNotFound("Post", reaction.PostId)
+		}
+
 		reaction.ChannelId = channelIds[0]
 	}
 	err = s.saveReactionAndUpdatePost(transaction, reaction)